1. Field of the Invention
The present invention relates to an improvement of a dust preventive structure employed in an air-cooled engine that is mounted mainly on a work machine such as, for example, a brush cutting machine as a drive source.
2. Description of the Prior Art
In general, an air-cooled engine has hitherto been employed as a drive source for a work machine such as, for example, a brush cutting machine. As best shown in FIG. 12, this known air-cooled engine is in the form of, for example, an vertical combustion engine having a rotary drive shaft 61 of the engine E1 oriented vertically and generally includes, in addition to the rotary drive shaft 61, a cooling fan assembly 62 drivingly coupled with the rotary drive shaft 61 and concurrently serving as a flywheel, and a fan casing 63 having an air intake opening 64 defined therein and so designed as to guide a stream of cooling air A1, induced by the cooling fan assembly 62, towards the air-cooled engine E1.
In this known brush cutting machine employing the air-cooled engine, in order to prevent dusts such as, for example, chips of turf and/or grass cut by the brush cutting machine, from intruding in admixture with the stream of cooling air A1, a dust preventive screen member 65 is disposed in the air intake opening 64 in the fan casing 63 and fitted to a recoil engagement 66 that is secured to the rotary drive shaft 61. This dust preventive screen member 65 has an outer peripheral portion formed with a cutter 67. The cutter 67 extends radially outwardly therefrom over the air intake opening 64 in the fan casing 63 and terminates in the vicinity of an outer peripheral surface 68a of a flange 68 defining an outer edge of the air intake opening 64. A recoil casing 69 concurrently serving as a covering is utilized to cover an outer area of the air intake opening 64 in the fan casing 63.
According to the Japanese Utility Model Registration No. 2513184, the brush cutting machine of the structure described above is so designed and so operable that long turf and/or grass 70 sucked by the cooling fan assembly 62 move radially outwardly of the screen member 65 by the effect of a centrifugal force developed by rotation of the screen member 65 and are, after having been cut by the cutter 67, discharged to the outside of the fan casing 63 by the effect of the centrifugal force.
The Japanese Utility Model Examined Publication No. 60-21466, published Jun. 26, 1985, discloses a similar brush cutting machine, but designed to have a structure for sucking chips of turf and/or grass after long turf and/or grass have been cut to the chips.
It has however been found that since the cutter 67 integral with the screen member 65 referred to above is positioned in the vicinity of the outer peripheral surface 68a of the flange 68 forming the air intake opening 64 in the fan casing 63, an overhang OH, i.e., the distance measured in a radial direction from a point of support of the screen member 65 to the outer peripheral edge of the screen member 65 where the cutter 67 is formed, tends to be large. For this reason, in mounting the screen member 65 which rotates together with the cooling fan assembly 62, the screen member 65 must be supported by a support structure of a design effective to secure a sufficient mounting rigidity and, accordingly, the freedom of design tends to be lowered.
Also, where the covering member is provided integrally with the fan casing 63 at the air intake opening 64, the screen member 65 in its entirety must be inserted into the fan casing 63. Accordingly, it is impossible to employ the structure, in which the cutter 67 integral with the screen member 65 is positioned in the vicinity of the outer peripheral surface 68a of the flange 68 forming the air intake opening 64.